Human Skin Pigmentation

Human skin has many functions, one of which is mediating the effects of sunlight. Pigmentation, (skin color), in humans has evolved under selection pressure from the duration and intensity of sunlight. Ancestral populations near the equator were selected for dark skin, while those living in higher northern latitudes were selected for lighter skin. The distribution of “light” alleles of well-studied genes associated with skin color is non-random. Some of these “light” versions are found in most northern latitude populations; others are more geographically specific.

Most of the known genes (and alleles) for skin color either 1) are part of the biosynthetic pathway for melanin, or 2) regulate melanin biosynthesis. Complicating the story is polygenic inheritance. The expression of the many genes that contribute to skin color results in a continuum of skin color. This continuum becomes more pronounced as populations migrate and interbreed. There are no measureable genetic differences among and between human populations. Thus, the notion of “race” is a societal idea, not a biological trait.

Both gene expression and natural selection are required for “building” traits of an organism, including skin color in humans. The outer cells of the skin provide protection from the sun by synthesizing and distributing pigment. Important in this process are melanocyte and keratinocyte cells.

Complicating the picture of gene expression is the many interacting genes/gene products involved in skin pigmentation. A simple model explains the influence that multiple genes can have on a characteristic. Complexity is compounded by other gene properties, such as multiple alleles and epistasis.

UV light was a strong factor in the evolution of skin color; darker skin protects cell nuclei from UV damage. Near the equator, this protection meant that populations with darker skin pigmentation had more offspring than populations with lighter skin pigmentation. Conversely, at higher latitudes, populations with lighter skin had more offspring (related to vitamin D production).

Within and between population genetic differences are used to characterize the distribution of alleles in five genes associated with skin color around the world. This information helps us understand the basis of the biogeographic distribution of skin colors.